1. Technical Field
The present invention relates to a liquid discharging apparatus, a head unit, a capacitive load driving circuit, and a control method of a capacitive load driving circuit.
2. Related Art
In a liquid discharging apparatus, such as an ink jet printer, which discharges ink and prints an image or a document, an apparatus which uses a piezoelectric element (for example, a piezo element) is known. The piezoelectric elements are provided corresponding to each of a plurality of nozzles in a head unit, and each of the piezoelectric elements is driven in accordance with driving signals. Accordingly, a predetermined amount of ink (liquid) is discharged from the nozzle at a predetermined timing, and a dot is formed. Since the piezoelectric element is a capacitive load, such as a capacitor, in terms of electricity, it is necessary to supply sufficient amount of current in order to operate the piezoelectric elements of each nozzle.
For this reason, in the above-described liquid discharging apparatus, the piezoelectric elements are driven as a driving signal which is amplified by an amplifying circuit is supplied to a head unit (ink jet head). An example of the amplifying circuit includes a type which performs current amplification with respect to a source signal before the amplification by using a class-AB amplifier, but since energy efficiency is not excellent, in recent years, a type in which a class-D amplifier is used has been suggested (refer to JP-A-2010-114711 and JPA-2005-329710).
Here, the invention of JP-A-2005-329710 includes a bootstrap circuit configured of a diode D0 and a capacitor C0. Then, in a class-D amplification path, an analog driving signal is demodulated by providing a low-pass filter (LPF) for returning to an original analog driving signal after power amplification. The LPF is, for example, configured of a coil and a capacitor, and drives an actuator configured of a capacitive load. Thus, a potential of a signal input portion of the LPF before the start of an operation of a gate driver is equal to a high potential supplied to the gate driver. That is, the potential of the signal input portion of the LPF is not a ground potential before the start of the operation of the gate driver as in a case of driving a resistive load.
Then, when the operation of the gate driver is started, a current sharply flows through a transistor (for example, corresponding to a transistor Q25 of FIG. 2 in JP-A-2005-329710) on a grounded side (hereinafter, low-side). Then, when the potential of the input portion of the LPF becomes the ground potential, a sharp current flows through the diode D0 to charge the capacitor C0 of the bootstrap circuit. In this case, when a current of more than rated current flows, there is a concern that the transistor Q25 and the diode D0 on the low-side are deteriorated.